Engines may be configured with boosting devices, such as turbochargers or superchargers, to increase mass airflow into a combustion chamber. Turbochargers and superchargers compress intake air entering the engine using an intake compressor. While a turbocharger includes a compressor that is driven by a turbine, a supercharger includes a compressor that is driven by the engine, or by a motor. In some engine systems, one or more intake charging devices may be staged in series or parallel to improve engine boost response.
One example of a multi-staged boosted engine is shown by Kawamura et al. in U.S. Pat. No. 6,938,420. Therein, an electric supercharger driven by an electric motor and an electric supercharger bypass valve (ESBV) are staged downstream of a turbocharger. During conditions when the turbocharger compressor is not spun up, the ESBV may be closed and the electric supercharger may be rotated to provide a transient positive boost pressure in order to reduce turbo lag. Then, when the turbocharger compressor is sufficiently spun up, the ESBV may be opened and the electric supercharger may be disabled, allowing the turbocharger to provide the desired boost pressure.
However the inventors herein have identified potential issues with such an approach. As one example, during selected vehicle maneuvers, such as cornering maneuvers, sliding maneuvers, maneuvers on sand or snow, etc., a larger amount of traction torque may be selectively added to front or rear tires. Typically, before such maneuvers, the vehicle may have undergone a deceleration event where vehicle brakes were applied and the acceleration pedal was released. Responsive to the deceleration event, engine actuators may have been adjusted to reduce boost pressure and move away from positions of best volumetric efficiency. For example, boost pressure may have been dumped by recirculating boosted air to a compressor inlet to mitigate surge, and an exhaust waste-gate may have been opened to decelerate a turbocharger turbine. As a result, when the driver tips back in shortly after the tip-out event, there is a perceptible transient torque deficiency which strongly affects the vehicle's maneuverability. A similar transient torque deficiency may occur when a vehicle launch is requested shortly after a deceleration event. Therein, the transient peak acceleration at the time of the vehicle launch (e.g., during the first second) may be limited, reducing the quality of the vehicle launch and driver satisfaction. Further, there may be NVH issues, such as a whoosh, experienced at the tip-in that degrades drivability. If boost pressure is reduced to address the tip-in whoosh, engine performance may be compromised.
In one example, the above issues may be at least partly addressed by a method for an engine comprising: responsive to a decrease in operator torque demand while operating in a drift mode, fully closing each of an exhaust waste-gate valve coupled to a turbocharger turbine, a recirculation valve coupled to a turbocharger compressor, and a bypass valve coupled to a supercharger compressor staged upstream of the turbocharger compressor; fully opening an intake throttle valve; and actuating an electric motor coupled to the supercharger compressor. In this way, engine torque and boost response may be improved even as the operator performs various sliding vehicle maneuvers.
As an example, an operator may select a drift mode of vehicle performance for a boosted vehicle (e.g., via a button) wherein the vehicle performance is geared to specific maneuvers. Responsive to the selection, one or more traction control settings of the vehicle may be adjusted to allow for torque vectoring wherein wheel torque is distributed unequally between front and rear wheels, as well as left and right side wheels. In addition, to improve the power output and boosted engine response of the vehicle in the drift mode, boost actuator settings may be adjusted. For example, independent of the operator demanded torque (e.g., independent of the accelerator pedal position), an exhaust waste-gate valve may be held closed to keep a turbocharger turbine spinning. In addition, a recirculation valve coupled to the turbocharger compressor may be held fully closed to keep spinning the compressor. Further, an electric motor coupled to a supercharger compressor, positioned upstream of the turbocharger compressor in the engine intake, may be actuated (e.g., to 100% duty cycle) to accelerate the supercharger compressor and meet any transient boost pressure requirements. Furthermore, intake and exhaust cams may be actuated to positions that provide the highest volumetric efficiency at the existing engine operating conditions. Engine torque may be then be adjusted to meet the operator torque demand via adjustments to spark timing and the opening of an intake throttle. For example, responsive to a drop in operator torque demand, spark timing is retarded, subject to spark authority and hardware constraints. If the drop in torque demand cannot be provided via (only) spark adjustments, engine torque may be reduced by decreasing the opening of the intake throttle and optionally disabling cylinder fueling. As another example, responsive to a rise in operator torque demand, spark timing may be moved towards maximum brake torque (MBT). If the rise in torque demand cannot be provided via (only) spark adjustments, engine torque may be increased by increasing the opening of the intake throttle. As a result of the air path actuator adjustments, the throttle inlet pressure may be maintained above barometric pressure regardless of the status of the turbocharger, thereby improving the transient torque response of the engine whenever the driver tips in. In addition, a positive feedback loop is created that further improves the turbocharger response.
In comparison, when the operator does not select the drift mode, responsive to a drop in operator torque demand, the exhaust waste-gate valve may be opened to rapidly spin down the turbine, the recirculation valve may be opened to rapidly spin down the compressor, and the intake throttle opening may be reduced to decrease airflow to the engine. Further, the electric motor may be maintained disabled and cylinder fueling may be disabled. As a result of the adjustments, manifold pressure may be rapidly dropped to barometric pressure conditions while the engine torque output is reduced. On a subsequent tip-in, the boost response may be slower, but that may be acceptable when not operating in the performance requiring mode.
In this way, engine torque and boost response may be improved when an operator is performing selected vehicle maneuvers. By holding an exhaust waste-gate valve and an intake compressor recirculation valve closed during selected performance modes, irrespective of whether an operator torque demand increases or decreases, an intake manifold pressure (e.g., upstream of an intake throttle) may be maintained elevated. Consequently, when there is a sudden increase in torque demand, boost pressure can be rapidly provided and engine torque can be rapidly increased. This allows a tip-in torque demand to be met as soon as it is demanded, improving vehicle maneuverability during the execution of the various vehicle maneuvers. In addition, vehicle launch times may be reduced, especially when a vehicle launch is requested shortly after a deceleration event. Furthermore, NVH issues associated with tip-in whoosh can be reduced without compromising engine performance. Overall, vehicle drivability is enhanced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.